Production method of ceramic honeycomb structure, and ceramic honeycomb structure

ABSTRACT

A method for producing a ceramic honeycomb structure comprising a ceramic honeycomb body having large numbers of longitudinal cells partitioned by porous cell walls having porosity of 50% or more, and a peripheral wall formed on a peripheral surface of the ceramic honeycomb body, comprising the steps of extruding moldable ceramic material to form a honeycomb-structured ceramic green body; machining a peripheral portion of the green body or a sintered body obtained from the green body to remove part of cell walls in the peripheral portion to obtain a ceramic honeycomb body having longitudinal grooves on a peripheral surface; applying colloidal metal oxide to a peripheral surface of the ceramic honeycomb body and drying it, and then applying a coating material comprising ceramic aggregate having an average particle size of 1 μm or more to form the peripheral wall.

FIELD OF THE INVENTION

The present invention relates to a method for producing a ceramichoneycomb structure, and a ceramic honeycomb structure.

BACKGROUND OF THE INVENTION

To remove toxic materials from exhaust gases discharged from internalcombustion engines of automobiles, etc., ceramic honeycomb structuresare used for exhaust-gas-cleaning catalyst converters, particulatematter (PM)-capturing filters, and carriers for catalysts for removingnitrogen oxides (NOx).

As shown in FIGS. 1( a) and 1(b), a ceramic honeycomb structure 1comprises a ceramic honeycomb body 10 having large numbers oflongitudinal cells 14 partitioned by porous cell walls 13, and aperipheral wall 11 formed on a periphery of the ceramic honeycomb body10, with a substantially circular or ellipsoidal cross sectionperpendicular to its flow paths [see FIG. 1( a)]. The ceramic honeycombstructure 1 is assembled in a metal container (not shown), in which itis fixed in a holding member (not shown) formed by a metal mesh orceramic mat, etc. Accordingly, the peripheral wall 11 should have enoughisostatic strength to withstand heat shock in a state where the ceramichoneycomb structure 1 is held by the holding member.

To reduce the amount of nitrogen oxides (NOx) contained in exhaust gasesof diesel engines, ceramic honeycomb structures carrying NOx catalystson cell walls are used. To provide the ceramic honeycomb structures withhigher NOx-cleaning capacity, it is effective to increase the amount ofa catalyst carried. To this end, cell walls should have as high porosityas, for example, 50% or more.

JP 05-269388 A discloses a ceramic honeycomb structure comprising aceramic honeycomb body having large numbers of longitudinal cellspartitioned by porous cell walls, and longitudinal grooves open on theperipheral surface, and a peripheral wall formed by a coating materialfilled in the grooves. This ceramic honeycomb structure is produced byforming a sintered ceramic honeycomb body integral with a peripheralwall by a known method, removing peripheral cells by grinding to form aceramic honeycomb body having grooves on the peripheral surface, fillingthe grooves on the peripheral surface with a coating material pastecomprising ceramic particles and/or ceramic fibers and colloidal silicaor colloidal alumina and drying it to form the peripheral wall. JP05-269388 A describes that such method produces a ceramic honeycombstructure having a reinforced peripheral portion, with excellent heatresistance and heat shock resistance.

However, when the peripheral wall described in JP 05-269388 A is formedin a ceramic honeycomb structure comprising cell walls having as highporosity as, for example, 50% or more, the peripheral wall fails toexhibit a sufficient strength-improving effect because of extremely lowstrength of the cell walls, resulting in a ceramic honeycomb sinteredbody failing to having enough isostatic strength to withstand heat shockduring use.

JP 2004-175654 A discloses a ceramic honeycomb structure comprising aceramic honeycomb body having large numbers of longitudinal cellspartitioned by porous cell walls and longitudinal grooves open on theperipheral surface, and a peripheral wall filling the grooves, withstress-releasing portions (voids) at least partially in the peripheralwall or between the peripheral wall and the grooves. JP 2004-175654 Adescribes that even if it were subject to heat shock, cracks due to heatshock would not easily propagate to cell walls, resulting in excellentheat shock resistance. This ceramic honeycomb structure is produced byforming a sintered ceramic honeycomb body integral with a peripheralwall by a known method, removing part of cell walls in the peripheralwall by machining to form a honeycomb body having grooves on aperipheral surface, applying a coating material comprising ceramicaggregate and an inorganic binder to substantially fill the grooves, andrapidly drying the coating material in a drying furnace at 70° C. orhigher.

However, because the ceramic honeycomb structure described in JP2004-175654 A has stress-releasing portions (crack-like voids open on anouter surface of the peripheral wall, or gaps between the ceramichoneycomb body and the peripheral wall), the peripheral wall is easilydetachable from the ceramic honeycomb body. Particularly when the methoddescribed in JP 2004-175654 A is used for the production of a ceramichoneycomb body comprising cell walls having as high porosity as, forexample, 50% or more, the resultant ceramic honeycomb body does not havesufficient isostatic strength.

JP 2006-255542 A discloses a ceramic honeycomb structure comprising acellular structure having pluralities of cells partitioned by porouscell walls, and an outer wall formed by a coating material comprisingceramic particles having an average particle size of 20-50 μm on aperipheral surface of the cellular structure, the porosity of the outerwall being smaller in an outer region than in an inner region in athickness direction. JP 2006-255542 A describes that the ceramichoneycomb structure has excellent durability and wear resistance becauseof little detachment of ceramic particles from the outer wall, and thatprintings on the outer wall surface are resistant to wear and damage.The ceramic honeycomb structure described in JP 2006-255542 A isproduced by removing a peripheral portion from a honeycomb-structuredsintered body formed by a known method by grinding, applying a coatingmaterial to the peripheral surface to form a peripheral coating layer,drying the peripheral coating layer completely or partially, andapplying a coating material comprising colloidal ceramic such ascolloidal silica, colloidal alumina, etc. as a main component to theperipheral coating layer to form a dense layer.

The peripheral wall described in JP 2006-255542 A, which has a porositygradient, has excellent wear resistance and damage resistance inprintings on the surface. However, when this peripheral coating layer isformed on a ceramic honeycomb body comprising cell walls having as highporosity as, for example, 50% or more, it is easily detached from theperipheral surface of the ceramic honeycomb body, because cell walls areextremely brittle. This means that the coated peripheral wall has pooradhesion to the ceramic honeycomb body.

JP 2003-284923 A discloses, as shown in FIG. 5, a ceramic honeycombstructure 50 comprising a ceramic honeycomb body 51, whose outermostcells and predetermined numbers of cells inside the outermost cells arecells 54 sealed by a peripheral wall 52 at one-side ends and/or inintermediate portions to prevent a fluid from flowing. JP 2003-284923 Adescribes that this ceramic honeycomb structure 50 can enhance theactivity of a catalyst carried in a short period of time, because of ashort temperature elevation time from the start of operation, which isachieved by the heat insulation of the sealed cells 54 formed by theperipheral wall 52. JP 2003-284923 A describes that this ceramichoneycomb structure 50 is produced by drying and sintering an extrudedhoneycomb-structured green body with different shrinkage ratios betweentwo ends to form a frustoconical ceramic honeycomb body 51, machining afrustoconical peripheral surface of the ceramic honeycomb body to acylindrical shape, and forming a peripheral wall 52 by a coatingmaterial such as a ceramic cement, etc. on a peripheral surface 51 athereof. JP 2003-284923 A lists cordierite, ceramic materials comprisingcordierite and/or ceramic fibers and an amorphous oxide matrix(colloidal silica, colloidal alumina, etc.), etc., as materials for theperipheral wall 52.

However, when the invention described in JP 2003-284923 A is applied toa ceramic honeycomb body comprising cell walls having as high porosityas, for example, 50% or more, the peripheral wall described in JP2003-284923 A cannot provide the ceramic honeycomb structure withsufficient isostatic strength, because cell walls are extremely brittle.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to solve the aboveproblems, providing a ceramic honeycomb structure having sufficientisostatic strength even though it comprises high-porosity cell walls,with a peripheral wall formed by a coating material being not easilydetached from the honeycomb structure.

DISCLOSURE OF THE INVENTION

As a result of intensive research in view of the above object, theinventors have found that the application of colloidal metal oxide and acoating material in this order to form a peripheral wall on a ceramichoneycomb body provides even a ceramic honeycomb body comprisinghigh-porosity cell walls with sufficient isostatic strength. The presentinvention has been completed based on such finding.

Thus, the method of the present invention for producing a ceramichoneycomb structure comprising a ceramic honeycomb body having largenumbers of longitudinal cells partitioned by porous cell walls havingporosity of 50% or more, and a peripheral wall formed on a peripheralsurface of the ceramic honeycomb body, comprises the steps of

extruding a moldable ceramic material to form a honeycomb-structuredceramic green body;

machining a peripheral portion of the green body or a sintered bodyobtained from the green body to remove part of cell walls in theperipheral portion, thereby obtaining a ceramic honeycomb body havinglongitudinal grooves on a peripheral surface;

coating a peripheral surface of the ceramic honeycomb body withcolloidal metal oxide and drying it; and

further coating the peripheral surface of the ceramic honeycomb bodywith a coating material comprising ceramic aggregate having an averageparticle size of 1 μm or more to form the peripheral wall.

The colloidal metal oxide is preferably colloidal silica or colloidalalumina.

The amount of the colloidal metal oxide coated is preferably2.0×10⁻³−150×10⁻³ g/cm³ on a solid basis, per a unit volume of theceramic honeycomb body.

The colloidal metal oxide preferably has an average particle size of5-100 nm.

The ceramic honeycomb structure of the present invention comprises aceramic honeycomb body having large numbers of longitudinal cellspartitioned by porous cell walls, and a peripheral wall formed on aperipheral surface of the ceramic honeycomb body,

the ceramic honeycomb body having longitudinal grooves on a peripheralsurface; and

the peripheral wall filling the longitudinal grooves, so that cell wallsconstituting the grooves on the peripheral surface have smaller porositythan that of cell walls in an inner portion of the ceramic honeycombbody.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a schematic view longitudinally showing one example ofceramic honeycomb structures produced by the method of the presentinvention.

FIG. 1( b) is a schematic, longitudinal cross-sectional view showing oneexample of ceramic honeycomb structures produced by the method of thepresent invention.

FIG. 2( a) is an enlarged schematic view showing part of an end surfaceof a ceramic honeycomb body produced by the method of the presentinvention.

FIG. 2( b) is an enlarged, longitudinal, partial cross-sectional viewshowing a ceramic honeycomb body produced by the method of the presentinvention.

FIG. 3( a) is a schematic view longitudinally showing grooves coatedwith colloidal metal oxide on a peripheral surface of the ceramichoneycomb body.

FIG. 3( b) is a longitudinal, partial cross-sectional view showinggrooves coated with colloidal metal oxide on a peripheral surface of theceramic honeycomb body.

FIG. 4( a) is a schematic view longitudinally showing grooves coatedwith colloidal metal oxide and a coating material on a peripheralsurface of the ceramic honeycomb body.

FIG. 4( b) is a longitudinal, partial cross-sectional view showinggrooves coated with colloidal metal oxide and a coating material on aperipheral surface of the ceramic honeycomb body.

FIG. 5 is a schematic cross-sectional view showing the ceramic honeycombstructure described in JP 2003-284923 A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be explained below,without intention of restricting the present invention thereto. Withinthe scope of the present invention, any modifications and improvements,etc. may be made properly based on the knowledge of those skilled in theart.

[1] PRODUCTION METHOD OF CERAMIC HONEYCOMB STRUCTURE

The method of the present invention for producing a ceramic honeycombstructure 1 comprising a ceramic honeycomb body 10 having large numbersof longitudinal cells 14 partitioned by porous cell walls 13 havingporosity of 50% or more, and a peripheral wall 11 formed on a peripheryof the ceramic honeycomb body 10 as shown in FIGS. 1( a) and 1(b),comprises the steps of

(a) extruding a moldable ceramic material to form a honeycomb-structuredceramic green body;(b) machining a peripheral portion of the green body or a sintered bodyobtained from the green body to remove part of cell walls in theperipheral portion, thereby obtaining a ceramic honeycomb body havinglongitudinal grooves on a peripheral surface; and(c) coating a peripheral surface of the ceramic honeycomb body withcolloidal metal oxide, and drying it, and further coating the peripheralsurface of the ceramic honeycomb body with a coating material comprisingceramic aggregate having an average particle size of 1 μm or more toform the peripheral wall.

(a) Formation of Green Body

The honeycomb-structured ceramic green body is produced by extruding amoldable ceramic material. Ceramic powder is fully dry-mixed with abinder, a lubricant, and if necessary, a pore-forming material, andsufficiently blended with water to form a plasticized ceramic material.This ceramic material is extruded, cut to a predetermined length, anddried to obtain a honeycomb-structured ceramic green body integrallycomprising a peripheral wall and cell walls.

(b) Production of Ceramic Honeycomb Body

The honeycomb-structured ceramic green body is sintered to provide asintered body having porosity of 50% or more. This sintered body isdeprived of part of cell walls in the peripheral portion by machining,to form a ceramic honeycomb body 10 having longitudinal grooves 140 on aperipheral surface 11 a as shown in FIGS. 2( a) and 2(b). Though aperipheral surface is machined after the green body is sintered in thisexample, the green body before sintering may be machined and thensintered to form the ceramic honeycomb body 10.

Materials preferable for the ceramic honeycomb body 10 includecordierite, alumina, silica, silicon nitride, silicon carbide, aluminumtitanate, LAS, etc. Among them, ceramic comprising cordierite as a maincrystal phase is most preferable because it is inexpensive and hasexcellent heat resistance and chemical stability.

(c) Formation of Peripheral Wall

(i) Application of Colloidal Metal Oxide

As shown in FIGS. 3( a) and 3(b), colloidal metal oxide 21 is applied togrooves 140 on a peripheral surface 11 a of the ceramic honeycomb body10 by a brush, a roller, etc. The colloidal metal oxide 21 appliedintrude into pores in cell walls 13 a constituting the grooves 140 onthe peripheral surface 11 a as well as in cell walls 13 b inside thecell walls 13 a, thereby clogging the pores to increase the strength ofcell walls 13 a on the peripheral surface 11 a. Colloidal metal oxide 21applied to the grooves 140 is dried by spontaneous drying, hot-airdrying in a drying furnace, etc.

The colloidal metal oxide 21 includes colloidal silica, colloidalalumina, titania sol, water glass, etc. Among them, colloidal silica andcolloidal alumina are preferable. The colloidal metal oxide 21 is usedpreferably in the form of an aqueous dispersion, with the concentrationof a solid component properly adjusted to have viscosity suitable forcoating.

The amount of colloidal metal oxide 21 coated is preferably 2.0×10⁻³g/cm³ to 150×10⁻³ g/cm³ on a solid basis, per a unit volume of theceramic honeycomb body 10. When the amount of the coating is less than2.0×10⁻³ g/cm³ on a solid basis less, pores in the cell walls 13 aconstituting the grooves 140 and in the cell walls 13 b inside the cellwalls 13 a may not be sufficiently clogged, failing to obtain sufficientisostatic strength. On the other hand, when it exceeds 150×10⁻³ g/cm³, alarge amount of colloidal metal oxide 21 fills the grooves 140,resulting in low heat shock resistance. The amount of the coating ispreferably 4.0×10⁻³ g/cm³ to 90×10⁻³ g/cm³ on a solid basis. The amount[g/cm³] of a solid component coated per a unit volume is a valueobtained by dividing the solid-basis amount [g] of colloidal metal oxide21 coated by the volume [cm³] of the ceramic honeycomb body, which isexpressed, for example, by [(π/4)×D²×L] in the case of a cylindricalshape having an outer diameter of D and a length of L.

The colloidal metal oxide 21 preferably has a particle size of 5-100 nm.The use of colloidal metal oxide 21 having a particle size in such arange makes it easy for the colloidal metal oxide 21 enter pores in thecell walls 13 a constituting the grooves 140, resulting in sufficientisostatic strength. The particle size of less than 5 nm undesirablydecreases heat shock resistance. On the other hand, the particle sizeexceeding 100 nm makes it difficult for the colloidal metal oxide toenter pores in the cell walls 13 a constituting the grooves 140,resulting in insufficient clogging of pores in the cell walls 13 a, andthus likely failing to obtain sufficient isostatic strength. Theparticle size is preferably 10-90 nm.

(ii) Application of Coating Material

As shown in FIGS. 4( a) and 4(b), a coating material 22 is applied in athickness of 0.1-3 mm onto the dried colloidal metal oxide 21 to fillthe grooves 140 on the peripheral surface 11 a of the ceramic honeycombbody 10. The applied coating material 22 is dried by a known method suchas hot-air drying, microwave drying, etc. to remove moisture in thecoating material 22, thereby obtaining a ceramic honeycomb structure 1having a peripheral wall 11.

The coating material 22 used is a paste obtained by blending ceramicaggregate having an average particle size of 1 μm or more, colloidalsilica or colloidal alumina, a binder, water, and if necessary, adispersant, ceramic fibers, etc. The use of ceramic aggregate having anaverage particle size of 1 μm or more in the coating material 22increases the strength of the peripheral wall 11, and thus the isostaticstrength of the ceramic honeycomb structure 1. However, when the averageparticle size of the ceramic aggregate is less than 1 μm, a large amountof colloidal silica or colloidal alumina is needed to bind the ceramicaggregate, providing the peripheral wall 11 with low heat shockresistance. On the other hand, ceramic aggregate having too large anaverage particle size provides the peripheral wall 11 with low strength,resulting in easy detachment of the peripheral wall 11 from the ceramichoneycomb structure. Accordingly, the average particle size of theceramic aggregate is preferably 2-50 μm.

Materials for the ceramic aggregate used in the coating material 22 maybe the same as or different from those of the ceramic honeycomb body 10,preferably cordierite, alumina, mullite, silica, etc. The use of amaterial having a smaller thermal expansion coefficient than that of theceramic honeycomb body 10 preferably provides high heat shock resistanceduring use. For example, amorphous silica is preferable.

When colloidal metal oxide 21 applied to the grooves 140 is the same asceramic aggregate in the coating material 22 applied thereon, thecolloidal metal oxide 21 and the coating material 22 preferably havegood adhesion to the cell walls 13 a, resulting in high bonding strengthbetween the cell walls 13 a and the peripheral wall 11.

In the ceramic honeycomb structure 1 produced by the method of thepresent invention, the colloidal metal oxide 21 applied to cell walls 13a constituting the longitudinal grooves 140 on the peripheral surface 11a intrude into pores in the cell walls 13 a as well as in the cell walls13 b inside the cell walls 13 a, and are strongly bonded thereto. As aresult, colloidal metal oxide 21 and the coating material 22 appliedthereon are integrated with the cell walls 13 a, resulting in highadhesion (bonding strength) between the cell walls 13 a and theperipheral wall 11. Accordingly, even a ceramic honeycomb structure 1comprising cell walls 13 having as high porosity as 50% or more hasextremely high isostatic strength because of a peripheral wall 11 formedby the colloidal metal oxide 21 and the coating material 22 appliedthereon, so that the peripheral wall 11 is not easily detached from theperipheral surface 11 a of the ceramic honeycomb structure.

When the method of the present invention is used to produce a ceramichoneycomb structure 50 having cells 54 whose ends on one side are sealedby a peripheral wall 52 as shown in FIG. 5, the detachment of theperipheral wall can be prevented, and cells sealed by the peripheralwall exhibit higher heat insulation, because colloidal metal oxideapplied to the grooves of the sealed cells 54 intrude into pores of cellwalls constituting the grooves, and because the pores of the cell wallsare clogged by the coating material applied thereon. Accordingly, theceramic honeycomb structure 50 enjoys such high-speed temperatureelevation that high catalytic activity can be achieved in a short periodof time from the start of operation.

[2] CERAMIC HONEYCOMB STRUCTURE

As shown in FIGS. 1( a), 1(b), 2(a) and 2(b), the ceramic honeycombstructure 1 of the present invention comprises a ceramic honeycomb body10 having large numbers of longitudinal cells 14 partitioned by porouscell walls 13, and a peripheral wall 11 formed on a peripheral surface11 a of the ceramic honeycomb body 10,

the ceramic honeycomb body 10 having longitudinal grooves 140 on theperipheral surface 11 a; and

the peripheral wall 11 being filled in the longitudinal grooves 140, sothat cell walls 13 a constituting the grooves 140 on the peripheralsurface 11 a have smaller porosity than that of cell walls 13 in aninner portion of the ceramic honeycomb body 10.

The porosity of cell walls 13 in the ceramic honeycomb structure 1 ispreferably 50% or more. Also, it is preferably 80% or less to providethe ceramic honeycomb structure 1 with sufficient strength. Because thecolloidal metal oxide 21 intrudes into cell walls 13 a constituting thegrooves 140 on the peripheral surface 11 a of the ceramic honeycomb body10, the porosity of the cell walls 13 a is smaller than the porosity ofcell walls 13 in an inner portion of the ceramic honeycomb body 10. Tohave sufficient isostatic strength, the porosity of cell walls 13 aconstituting the grooves 140 on the peripheral surface 11 a ispreferably 0.9 times or more, more preferably 0.8 times or more, theporosity of cell walls 13 in an inner portion of the ceramic honeycombbody 10. To prevent the deterioration of heat shock resistance, theporosity of cell walls 13 a constituting the grooves 140 on theperipheral surface 11 a is preferably 0.1 times or more the porosity ofcell walls 13 in an inner portion of the ceramic honeycomb body 10.

Because the colloidal metal oxide 21 enters not only pores of the cellwalls 13 a constituting the grooves 140 on the peripheral surface 11 aof the ceramic honeycomb body 10, but also pores of cell walls 13 binside the cell walls 13 a, the porosity of the inside cell walls 13 bis also preferably smaller than the porosity of the cell walls 13 in aninner portion of the ceramic honeycomb body 10, like the porosity ofcell walls 13 a constituting the grooves 140 of the peripheral surface11 a. Up to 20 cells below the peripheral surface 11 a preferably havecell walls 13 b having smaller porosity than that of cell walls 13 in aninner portion of the ceramic honeycomb body 10. More than 20 such cellslead to large pressure loss. The number of such cells is preferably upto 15, more preferably up to 10. The porosity of the cell walls 13 bpreferably becomes larger from the peripheral surface 11 a toward insidegradually or stepwise.

The cell walls 13 of the ceramic honeycomb body 10 preferably havethickness of 0.1-0.4 mm, and a cell pitch of 1-3 mm. The ceramichoneycomb body 10 with such structure more effectively exhibits highisostatic strength.

[3] EXAMPLES

The present invention will be explained in further detail by Examplesbelow, without intention of restricting the present invention thereto.

Example 1 (1) Production of Ceramic Honeycomb Body

Kaolin powder, talc powder, silica powder and alumina powder were mixedto form a cordierite-forming material powder comprising 50% by mass ofSiO₂, 36% by mass of Al₂O₃, and 14% by mass of MgO. This material powderwas fully dry-mixed with methylcellulose and hydroxypropylmethylcellulose as a binder, a lubricant, and a foamed resin as apore-forming material, and then sufficiently blended with water toprepare a plasticized ceramic material. This ceramic material wasextruded, cut to a predetermined length, and dried to obtain ahoneycomb-structured ceramic green body comprising a peripheral portionintegral with cell walls. This green body was sintered, and machined toremove part of cell walls from the peripheral portion, thereby obtaininga cordierite honeycomb body comprising longitudinal grooves on aperipheral surface, which had an outer diameter of 266 mm, a length of305 mm, a cell wall thickness of 0.3 mm, a cell pitch of 1.57 mm, and acell wall porosity of 61%.

(2) Formation of Peripheral Wall

An aqueous dispersion of colloidal silica (average particle size: 15 nm,and solid concentration: 20% by mass) as colloidal metal oxide wasapplied to a peripheral surface of each of the resultant four ceramichoneycomb bodies, in an amount of 20×10⁻³ g/cm³ on a solid basis per aunit volume of the ceramic honeycomb body. After the applied colloidalmetal oxide was dried at room temperature for 2 hours, it was coatedwith a coating material obtained by blending 100 parts by mass ofceramic aggregate (silica powder having an average particle size of 15μm) with 12 parts by mass of colloidal silica on a solid basis, and thenblending 100 parts by mass in total of ceramic aggregate and colloidalsilica with 1.2 parts by mass of methylcellulose together with water.The coated material was dried at 130° C. for 2 hours to produce fourceramic honeycomb structures.

The ceramic honeycomb structures were evaluated with respect toisostatic strength, and the adhesion and heat shock resistance of theperipheral wall. Also, with respect to the ceramic honeycomb structureused for the above evaluation, samples were cut out of cell walls in aninner portion of the ceramic honeycomb body, and cell walls in thegrooves on the peripheral surface, to measure their porosities bymercury porosimetry. Further, cell walls in the peripheral portion werecut out for observation by an electron microscope, to count how manycells having walls into which colloidal metal oxide intruded existedfrom below the peripheral surface toward inside.

Isostatic Strength

The isostatic strength test was conducted under the automobile standardsM505-87 of the Japanese Automotive Standards Organization (JASO). Bothends of the ceramic honeycomb structure were sealed by 20-mm-thickaluminum plates abutting its longitudinal both end surfaces, and a2-mm-thick rubber sheet was closely attached to a peripheral wallsurface of the ceramic honeycomb structure. This sample was placed in apressure-resistant container, into which water was introduced toisostatically exert pressure onto the peripheral wall surface. Thepressure at which the ceramic honeycomb structure was broken wasregarded as isostatic strength, which was evaluated by the followingstandards. The results are shown in Table 1.

Excellent: The isostatic strength was 2 MPa or more.

Good: The isostatic strength was 1.5 MPa or more and less than 2 MPa.

Fair: The isostatic strength was 1.0 MPa or more and less than 1.5 MPa.

Poor: The isostatic strength was less than 1.0 MPa.

Adhesion

The ceramic honeycomb structure was cut at three arbitrary pointsperpendicularly to the longitudinal direction, and the resultant threecross sections were observed by the naked eye with respect to gapsbetween the peripheral wall and the ceramic honeycomb body, therebyevaluating the adhesion of the peripheral wall to the ceramic honeycombbody by the following standards.

Excellent: There were no gaps in all of three cross sections.

Good: There were one or more gaps in one of three cross sections.

Fair: There were one or more gaps in two of three cross sections.

Poor: There were one or more gaps in all of three cross sections.

Heat Shock Resistance

Three ceramic honeycomb structures were heated at a constant temperaturefor 30 minutes in an electric furnace, and then rapidly cooled to roomtemperature to observe cracks by the naked eye for the evaluation ofheat shock resistance. Because cracking occurs more at a highertemperature, the heating temperature was elevated by every 25° C. untilcracking occurred. The same evaluation was conducted on three samples,and the difference between the heating temperature and room temperaturein a sample, in which cracking occurred at the lowest temperature amongthe three samples, was regarded as a heat shock resistance temperature.The heat shock resistance temperature was evaluated by the followingstandards.

Excellent: The heat shock resistance temperature was 600° C. or higher.

Good: The heat shock resistance temperature was 550° C. or higher andlower than 600° C.

Fair: The heat shock resistance temperature was 500° C. or higher andlower than 550° C.

Poor: The heat shock resistance temperature was lower than 500° C.

Examples 2-15 and Comparative Examples 3-7

Ceramic honeycomb structures were produced in the same manner as inExample 1, except for changing the type and solid-base amount ofcolloidal metal oxides applied, and the type and average particle sizeof ceramic aggregates in the coating material as shown in Table 1, andtheir isostatic strength, peripheral wall adhesion, and heat shockresistance were evaluated. The colloidal metal oxide was used in theseExamples in the form of an aqueous dispersion having a solidconcentration of 20% by mass.

Comparative Example 1

A ceramic honeycomb structure was produced in the same manner as inExample 1, except that a coating material obtained by changing theaverage particle size of silica powder used as ceramic aggregate from 15μm to 20 μm was directly applied to a peripheral surface of the ceramichoneycomb body without applying colloidal metal oxide, and its isostaticstrength, peripheral wall adhesion, and heat shock resistance wereevaluated.

Comparative Example 2

A ceramic honeycomb structure was produced in the same manner as inComparative Example 1, except that after a coating material directlyapplied to a peripheral surface of the ceramic honeycomb body was dried,colloidal silica (average particle size: 20 nm, and solid concentration:20% by mass) as colloidal metal oxide was applied thereonto in an amountof 50×10⁻³ g/cm³ on a solid basis per a unit volume of the ceramichoneycomb body, and then dried, and its isostatic strength, peripheralwall adhesion, and heat shock resistance were evaluated.

TABLE 1 Colloidal Metal Oxide Applied to Grooves on Peripheral SurfaceAmount of Solid Average Component Coated⁽¹⁾ Particle No. Type (×10⁻³g/cm³) Size (nm) Example 1 Colloidal Silica 20 15 Example 2 ColloidalSilica 20 20 Example 3 Colloidal Alumina 20 25 Example 4 ColloidalSilica 20 100 Example 5 Colloidal Silica 20 5 Example 6 Colloidal Silica150 20 Example 7 Colloidal Silica 2.0 20 Example 8 Colloidal Silica 5060 Example 9 Colloidal Silica 50 2 Example 10 Colloidal Silica 155 20Example 11 Colloidal Silica 1.0 20 Example 12 Colloidal Silica 20 20Example 13 Colloidal Silica 20 20 Example 14 Colloidal Silica 20 20Example 15 Colloidal Alumina 20 20 Com. Ex. 1 — — — Com. Ex. 2 — — —Com. Ex. 3 Colloidal Silica 50 20 Com. Ex. 4 Colloidal Silica 50 200Com. Ex. 5 Colloidal Silica 50 2 Com. Ex. 6 Colloidal Silica 160 20 Com.Ex. 7 Colloidal Silica 1.0 20 Note: ⁽¹⁾The amount of a solid componentcoated per a unit volume of a ceramic honeycomb body. Coating MaterialEvaluation Results Average Porosity of Type of Particle Cell Walls (%)Ceramic Size Inner No. Aggregate (μm) Portion⁽²⁾ Grooves⁽³⁾ Example 1Silica 15 60 15 Example 2 Silica 15 58 20 Example 3 Silica 15 59 22Example 4 Silica 15 59 46 Example 5 Silica 15 58 8 Example 6 Silica 1558 15 Example 7 Silica 20 58 30 Example 8 Silica 20 58 25 Example 9Silica 20 58 5 Example 10 Silica 20 58 10 Example 11 Silica 20 60 35Example 12 Cordierite 50 60 22 Example 13 Cordierite 80 60 22 Example 14Cordierite 1 60 22 Example 15 Cordierite 15 75 30 Com. Ex. 1 Silica 2060 55 Com. Ex. 2⁽⁴⁾ Silica 20 60 55 Com. Ex. 3 Silica 0.5 60 20 Com. Ex.4 Silica 0.5 60 30 Com. Ex. 5 Silica 0.5 60 10 Com. Ex. 6 Silica 0.5 6020 Com. Ex. 7 Silica 0.5 60 20 Note: ⁽²⁾Cells in the inner portion ofthe ceramic honeycomb body. ⁽³⁾Cells in grooves on the peripheralsurface. ⁽⁴⁾After the coated material was dried, colloidal metal oxidewas applied. Evaluation Results Number of Intruded Isostatic Heat ShockNo. Cells⁽¹⁾ Strength Adhesion Resistance Example 1 16 ExcellentExcellent Excellent Example 2 15 Excellent Excellent Excellent Example 314 Excellent Good Excellent Example 4 12 Good Excellent ExcellentExample 5 18 Excellent Excellent Good Example 6 25 Excellent ExcellentExcellent Example 7 9 Excellent Excellent Excellent Example 8 19 GoodExcellent Excellent Example 9 21 Excellent Excellent Fair Example 10 27Excellent Excellent Good Example 11 7 Good Good Good Example 12 15 GoodGood Good Example 13 15 Fair Fair Good Example 14 15 Excellent Good FairExample 15 15 Good Good Good Com. Ex. 1 — Poor Poor Poor Com. Ex. 2 —Poor Poor Poor Com. Ex. 3 20 Excellent Excellent Poor Com. Ex. 4 15 FairExcellent Poor Com. Ex. 5 25 Excellent Excellent Poor Com. Ex. 6 30Excellent Excellent Poor Com. Ex. 7 7 Good Good Poor Note: ⁽¹⁾The numberof cells into which colloidal metal oxide intruded.

As is clear from Table 1, the ceramic honeycomb structures of Examples1-15 each obtained by applying colloidal metal oxide to grooves on aperipheral surface and then applying a coating material comprisingceramic particles having an average particle size of 1 μm or more ontothe colloidal metal oxide layer had sufficient isostatic strength,peripheral wall adhesion and heat shock resistance. On the other hand,Comparative Example 1, in which the above coating material was directlyapplied to grooves on a peripheral surface without applying colloidalmetal oxide, did not have sufficient isostatic strength, and was low inperipheral wall adhesion and heat shock resistance. Like ComparativeExample 1, Comparative Example 2, in which the coating material wasdirectly applied to grooves on a peripheral surface, and colloidal metaloxide was applied thereonto, did not have sufficient isostatic strength.The ceramic honeycomb structures of Comparative Examples 3-7 eachobtained by applying colloidal metal oxide to grooves on a peripheralsurface, and applying a coating material comprising ceramic particleshaving an average particle size of less than 1 μm thereonto had poorheat shock resistance, despite relatively good isostatic strength andperipheral wall adhesion.

EFFECT OF THE INVENTION

Because the present invention provides a ceramic honeycomb structurehaving sufficient isostatic strength even when its cell walls have ashigh porosity as 50% or more, the peripheral wall is unlikely detachedfrom the ceramic honeycomb body even under heat shock during use.

1. A method for producing a ceramic honeycomb structure comprising a ceramic honeycomb body having large numbers of longitudinal cells partitioned by porous cell walls having porosity of 50% or more, and a peripheral wall formed on a peripheral surface of said ceramic honeycomb body, comprising the steps of extruding a moldable ceramic material to form a honeycomb-structured ceramic green body; machining a peripheral portion of said green body or a sintered body obtained from said green body to remove part of cell walls in the peripheral portion, thereby obtaining a ceramic honeycomb body having longitudinal grooves on a peripheral surface; coating a peripheral surface of said ceramic honeycomb body with colloidal metal oxide and drying it; and further coating the peripheral surface of said ceramic honeycomb body with a coating material comprising ceramic aggregate having an average particle size of 1 μm or more to form said peripheral wall.
 2. The method for producing a ceramic honeycomb structure according to claim 1, wherein said colloidal metal oxide is colloidal silica or colloidal alumina.
 3. The method for producing a ceramic honeycomb structure according to claim 1, wherein the amount of said colloidal metal oxide coated is 2.0×10⁻³−150×10⁻³ g/cm³ on a solid basis, per a unit volume of said ceramic honeycomb body.
 4. The method for producing a ceramic honeycomb structure according to claim 1, wherein said colloidal metal oxide has an average particle size of 5-100 nm.
 5. A ceramic honeycomb structure comprising a ceramic honeycomb body having large numbers of longitudinal cells partitioned by porous cell walls, and a peripheral wall formed on a peripheral surface of said ceramic honeycomb body, wherein said ceramic honeycomb body has longitudinal grooves on a peripheral surface; and wherein said peripheral wall fills said longitudinal grooves, so that cell walls constituting said grooves on the peripheral surface have smaller porosity than that of cell walls in an inner portion of said ceramic honeycomb body. 